#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N = 1e6 + 10;
int op;
vector<int> z;
bool st[N];
void init()
{
    for (int i = 2; i < N; i++)
    {
        if (st[i])
            continue;
        z.push_back(i);
        for (int j = i + i; j < N; j += i)
        {
            st[j] = 1;
        }
    }
}
void solve()
{
    int n;
    cin >> n;
    vector<int> v(n);
    int temp;
    for (int i = 0; i < n; i++)
    {
        cin >> v[i];
        if (i == 0)
            temp = v[i];
        else
            temp = gcd(temp, v[i]);
    }
    if (temp > 1)
    {
        cout << "Case " << ++op << ": " << 0 << endl;
        return;
    }
    sort(v.begin(), v.end());
    auto it = std::unique(v.begin(), v.end());
    v.erase(it, v.end());
    if (v.size() == 1)
    {
        if (v[0] == 1)
            cout << "Case " << ++op << ": " << 1 << endl;
        else
            cout << "Case " << ++op << ": " << 0 << endl;
        return;
    }
    int gc;
    for (int i = 1; i < v.size(); i++)
    {
        if (i == 1)
            gc = v[i] - v[i - 1];
        else
            gc = gcd(gc, v[i] - v[i - 1]);
    }
    int res;
    if (gc == 1)
        res = -1;
    else
    {
        for (int o : z)
        {
            if (gc % o == 0)
            {
                gc = o;
                break;
            }
        }
        res = gc - (v.back() % gc);
    }
    cout << "Case " << ++op << ": " << res << endl;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    init();
    int t = 1;
    cin >> t;
    while (t--)
        solve();
}